I have my 4 wt 8.5 foot BVK lined with 5 wt Rio Gold. I am not a great distance caster but I can really get it out there with that rod and it's current set up. Not sure how it would cast with 4 wt line.

I've only over-loaded 1 rod (from 5wt to 6wt) but it was an extremely stiff rod that could handle it. If you went back to 4 wt you would actually be able to cast it farther.